Usually, a four-stroke cycle engine to be mounted upon a vehicle such as, for example, an automobile and a motorcycle is provided with intake-exhaust valves above the combustion chamber thereof. These valves are driven by means of a valve actuating mechanism. Specifically, the valve actuating mechanism is provided with a cam shaft which is operated in association with the crankshaft of the engine so that the intake-exhaust valves are caused to move in an up and down mode at a predetermined time by means of a cam which is formed upon such cam shaft.
It is desirable for a four-stroke cycle engine that a high output may be obtained for a broad speed range extending from a low speed range to an intermediate-high speed range, that is, that the power band be wide.
In a conventional valve actuating mechanism, however, since the timing for the opening-closing of a valve and the amount of the lift are fixed, only an output characteristic having a peak value within a specific engine speed range may be obtained and one is forced to make a choice as to whether the output characteristic within the low speed region is to be emphasized or the output characteristic within the intermediate-high speed range is to be emphasized.